The several anticonvulsant drugs marketed in the United States provide significant seizure relief for only 50-75% of epileptic patients. The therapeutic effects are sometimes accompanied by serious side effects such as sedation, ataxia, psychoses, suicidal depression, gastrointestinal disturbances, gingival hyperplasia, lymphadenopathies, megaloblastic anemias, hepatotoxicity, nephropathies, hirsutism, and fetal malformations. These side effects, which range in severity from mild sedation to death from aplastic anemia, are particularly troublesome since most of the marketed anticonvulsants have very low therapeutic ratios. For example, phenytoin, one of the most widely used anticonvulsants, controls seizures in man only when plasma levels reach 10 mcg./ml. Toxic effects such as nystagmus are seen at around 20 mcg./ml., ataxia is obvious at 30 mcg./ml., and lethargy is apparent at about 40 mcg./ml. See "The Pharmacological Basis of Therapeutics" (Gilman, Goodman, and Gilman, ed., 6th Ed., MacMillan Publishing Co., Inc., New York, N.Y. (1980)), p. 455. In view of these facts, most epileptologists indicate there is a definite need for more selective and less toxic anticonvulsant drugs.
U.S. Pat. No. 4,379,165 claims a method of treating epilepsy and other convulsive disorders by administering certain N-substituted amino-benzamide derivatives. One particularly preferred compound taught in this patent is 4-amino-N-(.alpha.-methylbenzyl)benzamide, also known as 4-amino-N-(1-phenylethyl)benzamide. The compound is reported to have a protective index (P.I.) of 9.48, which is the ratio of the TD.sub.50 (the toxic dose in 50% of the subjects according to the Rotorod toxicity test) to the ED.sub.50 in the maximal electroshock seizure test (i.e., the effective dose required to abolish the hind-limb tonic-extensor component in 50% of the animals). The patent only discloses and refers to the compound in its racemic form and does not disclose or infer that the compound can exist as separate enantiomers.